1. Field of the Invention
This invention relates generally to marine seismic surveying, and, more particularly, to pre-stack combining of over/under seismic data acquired in a marine seismic survey.
2. Description of the Related Art
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. Since many commercially valuable hydrocarbon deposits are located beneath bodies of water, various types of marine seismic surveys have been developed. In a typical marine seismic survey, such as the exemplary survey 100 conceptually illustrated in FIG. 1, marine seismic streamers 105(1) or alternatively 105(2) are towed behind a survey vessel 110. The seismic streamers 105(1-2) may be several thousand meters long and contain a large number of sensors 115, such as hydrophones and associated electronic equipment, which are distributed along the length of the each seismic streamer cable 105(1-5). The survey vessel 110 also includes one or more seismic sources 120, such as airguns and the like.
As the streamers 105(1) or 105(2) are towed behind the survey vessel 110, acoustic signals 125, commonly referred to as “shots,” produced by the seismic source 120 are directed down through the water column 130 into strata 135, 140 beneath a water bottom surface 145, where they are reflected from the various subterranean geological formations 150. Reflected signals 155 are received by the sensors 115 in the seismic streamer cables 105(1-2), digitized, and then transmitted to the survey vessel 110. The digitized signals are referred to as “traces” and are recorded and at least partially processed by a signal processing unit 160 deployed on the survey vessel 110. The ultimate aim of this process is to build up a representation of the subterranean geological formations 150 beneath the streamers 105(1-2). Analysis of the representation may indicate probable locations of hydrocarbon deposits in the subterranean geological formations 150.
During a marine seismic survey, the high-frequency content of the acquired seismic data may be increased by deploying the streamer 105(1) at a shallow depth relative to a surface 165 of the water column 130. However, the low-frequency content of the acquired seismic data, which may be important for stratigraphic and/or structural inversion, may be attenuated when the streamer 105(1) is deployed at the shallow depth. Thus, the low-frequency content of the acquired seismic data may be enhanced by alternatively deploying the streamer 105(2) further beneath the surface 165. However, this approach enhances the low-frequency content at the expense of the high-frequency content of the seismic data.
Some of the advantages of deploying the streamer 105(1) at the shallow depth and some of the advantages of deploying the streamer 105(2) at a greater depth may be realized in a marine seismic survey that includes both the streamer 105(1) deployed at the shallow depth and the streamer 105(2) deployed at the greater depth, as shown in FIG. 1. For example, the streamer 105(1) may be deployed at a depth of 6 meters and the streamer 105(2) may be deployed at a depth of 9 meters. This arrangement of the streamers 105 (1-2) is sometimes referred to as an over/under combination of the streamers 105(1-2). The term “over” is typically associated with the shallow streamer 105(1) and the term “under” is typically associated with the deep streamer 105(2). The over/under combination technique is also known as a dual-streamer de-ghosting technique, an acoustic wave field decomposition, and the like. Moreover, the vertically-separated seismic sensors 115 may be referred to as a vertical receiver array.
FIG. 2 conceptually illustrates a conventional system 200 that may be used to perform a marine seismic survey using an over/under combination technique. The system 200 includes a survey vessel 205, which tows a shallow streamer 210(1) and a deep streamer 210(2). The shallow and deep streamers 210(1-2) each include at least one receiver 220(1-2). A source 215 provides a seismic signal 225 that is received by receivers 220(1-2). As indicated in FIG. 2, the source 215 is typically deployed at a different depth than the receivers 220(1-2). One or more ghost signals 230(1-2) are also received by the receivers 220(1-2). Thus, seismic data acquired by the receivers 220(1-2) includes contributions from at least the seismic signal 225(1-2) and the one or more ghost signals 230(1-2).
FIGS. 3A-D conceptually illustrate received seismic signals. In particular, FIGS. 3A and 3B conceptually illustrate a seismic signal that may be received by the shallow streamer 210(1) as a function of time (in FIG. 3A) and as a function of frequency (in FIG. 3B). As shown in FIG. 3A, the seismic signal includes an up-going wave field 310, which is approximately a delta-function corresponding to a flat amplitude spectrum seismic signal 315 in the frequency domain shown in FIG. 3B. A down-going wave field 320, corresponding to a ghost signal, is depicted in FIG. 3A as an approximate delta function with a negative amplitude that arrives at a later time than the up-going wave field 310. The “over” recorded seismic data 325 acquired by the shallow streamer 210(1) is a combination of the up-going wave field 310 and the down-going wave field 320. Accordingly, the “over” recorded seismic data 325 may include one or more notches 330 that may not be present in the flat amplitude spectrum seismic signal 315.
FIGS. 3C and 3D conceptually illustrate a seismic signal that may be received by the deep streamer 210(2) as a function of time (in FIG. 3C) and as a function of frequency (in FIG. 3D). As shown in FIG. 3C, the seismic signal includes an up-going wave field 350, which is approximately a delta-function corresponding to a flat amplitude spectrum seismic signal 355 in the frequency domain shown in FIG. 3D, and a down-going wave field 360, corresponding to a ghost signal, which is depicted in FIG. 3C as an approximate delta function with a negative amplitude that arrives at a later time than the up-going wave field 350. The “under” recorded seismic data 365 acquired by the source 215(2) on the deep streamer 210(2) includes one or more notches 370 that may not be present in the flat amplitude spectrum seismic signal 355.
The notches 330, 370 may result in resolution loss in the acquired seismic data. Thus, over/under combination technique attempts to estimate the up-going and down-going wave fields 310, 350 and 320, 360 by combining the “over” recorded data 325 and the “under” recorded data 365. For example, the up-going wave field 350 and a down-going wave field 360 of the deep streamer 210(2) are separated by a different time lag than the up-going wave field 310 and the down-going wave field 320 of the shallow streamer 210(1). The location of the notches 330, 370 depends on the depth of the streamers 210(1-2) and, consequently, the frequencies of the notches 370 are different than the frequencies of the notches 330. This property may be used to combine the “over” and “under” recorded data 325, 365 to reduce the effect of the notches 330, 370 in the combined data set.
However, conventional over/under data processing techniques do not account for variations in acquisition parameters, acquisition perturbations, sea height and non-ideal reflectivity, noise, streamer positioning errors, and the like that may reduce the quality of the over/under combination. For example, conventional techniques typically assume a nominal acquisition geometry in which the seismic cables are at a constant depth and are deployed precisely above one another. For another example, one conventional technique further assumes that the sea surface is a flat perfect reflector and applies the corresponding flat sea boundary condition to separate the up-going and down-going wave fields 310, 320, 350, 360, e.g. this conventional over/under data processing technique assumes boundary conditions corresponding to a surface reflectivity of −1 and a 180° phase difference between the up-going and down-going wave fields 310, 320, 350, 360.
In some conventional over/under data processing techniques, the “over” recorded data 325 may be combined with the “under” recorded data 365 during post-stack processing of the seismic data. In this embodiment, the over/under combination assumes vertical propagation of the various seismic signals. Stacking the recorded data 325, 365 typically applies a normal-move-out correction to bring the seismic signals to a common midpoint. However, assuming vertical propagation and/or applying the normal-move-out correction and stacking may not account for spatial and/or temporal variations in water velocity, ghost travel times, sea-bottom geometry, and the like. These assumptions can therefore reduce the quality of the over/under combination.
The present invention is directed to addressing the effects of one or more of the problems set forth above.